1. Technical Field
The present art relates to a transmission system and a transmission method thereof for transmitting a first optical signal modulated by first transmitter and a second optical signal modulated by a second transmitter via the same transmission line. In particular, the present art relates to a transmission system and a transmission method for preventing the waveform degradation of the optical signal even if mixing a plurality of optical signals based on different modulations (phase modulation and intensity modulation).
2. Description of the Related Art
Recently, the demands for introducing a 40 Gbit/s optical transmission system of the next generation are increased, and the transmission distance and frequency using efficiency equivalent to those of a 10 Gbit/s system are required. As realizing means, RZ-DPSK (Return-to-Zero Differential Phase-Shift Keying) modulation or CSRZ-DPSK (Carrier Suppressed Return-to-Zero Differential Phase-Shift Keying) modulation is greatly researched and developed, with the excellent tolerance of Optical Signal-to-Noise Ratio (OSNR) and Nonlinear tolerance higher than those of NRZ (Non Return to Zero) modulation applied to a conventional system of 10 Gbit/s or less.
In addition to the above modulations, as disclosed in Mos. 3. 2. and 6, ECCC 2006, “Nonlinear interaction between 10 Gbit/s NRZ channels and 40 Gbit/s channels with RZ-DQPSK or PSBT format over low-dispersion fiber”, written by G. Charlet et al., phase modulation such as RZ-DQPSK (Differential Quadrature Phase-Shift Keying) modulation having the feature of a narrow spectrum (high spectral utilization efficiency) or CSRZ-DQPSK modulation is also greatly researched and developed.
FIG. 27 is a diagram showing an example of the structure of an optical transmitting apparatus and an optical receiving apparatus using RZ-DPSK or CSRZ-DPSK modulation with 43 Gbit/s. Further, FIG. 28 is a diagram showing the optical intensity and the optical phase upon transmitting and receiving an RZ-DPSK or CSRZ-DPSK modulated optical signal.
Referring to FIG. 27, an optical transmitting apparatus 10 transmits an optical signal subjected to an RZ-DPSK or CSRZ-DPSK modulation with 43 Gbit/s, and comprises a transmitting data processing part 11, a CW (Continuous Wave) light source 12, a phase modulator 13, and an intensity modulator 14 for making RZ pulse.
Specifically, the transmitting data processing part 11 has a function as a framer that sets a frame of input data, a function as an FEC (Forward Error Correction) encoder that adds error correction code, and a function of a DPSK pre-coder, that performs coding processing for reflecting difference information between the current code and code before one bit thereof.
The phase modulator 13 modulates the phase of continuous light from the CW light source 12 in accordance with coding data from the transmitting data processing part 11, and outputs an optical signal having information on a binary optical phase although the optical intensity is constant, i.e., DPSK modulated optical signal (refer to the bottom in FIG. 28).
The intensity modulator 14 for making RZ pulse sets RZ pulses of the optical signal from the phase modulator 13 (refer to the top in FIG. 28). In particular, an RZ-DPSK signal denotes an optical signal that is set to RZ pulses with a clock drive signal having the same frequency (43 GHz) as that of a bit rate of data and a one-time amplitude of an on-off driving voltage (Vπ). Further, a CSRZ-DPSK signal denotes an optical signal that is set to RZ pulses with a clock drive signal having the half frequency (21.5 GHz) of the bit rate of data and a double amplitude of the on-off driving voltage (Vπ).
Moreover, an optical receiving apparatus 30 is connected to the optical transmitting apparatus 10 via a transmission line 20 and an optical repeater 21, and performs receiving signal processing of the (CS) RZ-DPSK signal via optical repeating transmission from the optical transmitting apparatus 10. For example, the optical receiving apparatus 30 comprises a delay interferometer 31, a photo-electronic converting part 32, a regeneration circuit 33, and a receiving data processing part 34.
Specifically, the delay interferometer 31 comprises an Mach-Zehnder interferometer, performs delay interference between a delay component (23.3 ps in the structure example in FIG. 27) corresponding to one-bit time and a component subjected to 0rad phase control of the (CS) RZ-DPSK signal transmitted via the transmission line 20, and outputs the interference result as two signals. Incidentally, the Mach-Zehnder interferometer is formed so that one division waveguide is longer than another division waveguide by a propagation length corresponding to one-bit time. An electrode is formed to control the phase of an optical signal that is propagated through the other division waveguide.
The photo-electronic converting part 32 comprises a dual-pin photodiode that receives the outputs from the delay interferometer 31 and thus performs balanced detection. Incidentally, the receiving signal detected by the photo-electronic converting part 32 is properly amplified by an amplifier.
The regeneration circuit 33 extracts a data signal and a clock signal from the receiving signal subjected to the balanced detection in the photo-electronic converting part 32. The receiving data processing part 34 performs signal processing such as error correction on the basis of the data signal and the clock signal extracted by the regeneration circuit 33.
FIG. 29 is a diagram showing an example of the structure of the optical transmitting apparatus and the optical receiving apparatus using the RZ-DQPSK or CSRZ-DQPSK modulation with 43 Gbit/s. FIG. 30 is a diagram showing the optical intensity and the optical phase upon transmitting and receiving the optical signal subjected to the RZ-DQPSK or CSRZ-DQPSK modulation.
Referring to FIG. 29, an optical transmitting apparatus 40 comprises a transmitting data processing part 41, a (1:2) demultiplexer (DEMUX) 42, a CW light source 43, a n/2-phase shifter 44, two phase shifters 45A and 45B, and an intensity modulator 46 for making RZ pulse.
Specifically, similarly to the transmitting data processing part 11 shown in FIG. 27, the transmitting data processing part 41 has functions of a framer and an FEC encoder, and further has a function of a DQPSK pre-coder that performs coding processing for reflecting difference information between the current core and code before one bit thereof.
The (1:2) demultiplexer 42 splits the coding data with 43 Gbit/s from the transmitting data processing part 41 into coding data #1 and #2 on two-series with 21.5 Gbit/s. The CW light source 43 outputs continuous light, the output continues light is split into two parts, one light is input to the phase shifter 45A, and the other light is input to the phase shifter 45B via the π/2 phase shifter 44.
The phase shifter 45A modulates the continuous light from the CW light source 43 with the coding data #1 on one of the two-series split by the (1:2) demultiplexer 42, and outputs an optical signal having information on binary optical phase (0rad or π rad). The phase shifter 45B receives light obtained by shifting the phase of the continuous light from the CW light source 43 with π/2 by the π/2 phase shifter 44, modulates the input light by the coding light #2 on the other-series split by the (1:2) demultiplexer 42, and outputs an optical signal having information on a binary optical phase (π/2 rad or 3π/2 rad).
The light modulated by the phase shifters 45A and 45B is coupled and is thereafter output to the intensity modulator 46 for making RZ pulse at the latter stage. That is, the modulation light from the phase shifters 45A and 45B is coupled, thereby transmitting, to the intensity modulator 46 for making RZ pulse, an optical signal having information on a four-bit optical phase although the optical intensity is constant (refer to the bottom in FIG. 30), that is, the optical signal subjected to the DQPSK modulation.
Similarly to the intensity modulator 14 for making RZ pulse shown in FIG. 27, the intensity modulator 46 for making RZ pulse sets the DQPSK-modulated optical signals from the phase shifters 45A and 45B to RZ pulses. In particular, an RZ-DQPSK signal denotes an optical signal that is set to RZ pulses with a clock drive signal having the same frequency (21.5 GHz) of that of the bit rate of the data #1 and #2 and a one-time amplitude of an on-off driving voltage (Vπ). A CSRZ-DQPSK signal denotes an optical signal that is set to RZ pulses with a clock drive signal having the half frequency (10.75 GHz) of that of the bit rate of the data #1 and #2 and a double amplitude of the on-off driving voltage (Vπ).
An optical receiving apparatus 60 is connected to the optical transmitting apparatus 40 via a transmission line 50 and an optical repeater 51, and performs receiving signal processing of the (CS) RZ-DQPSK signal transmitted via the optical repeating from the optical transmitting apparatus 40. The optical receiving apparatus 60 comprises a branch part 61 that branches the received optical signal into two parts, and delay interferometers 62A and 62B, photo-electronic converting parts 63A and 63B, and regeneration circuits 64A and 64B, which are on the branched optical signal lines for propagating the optical signals. Further, the optical receiving apparatus 60 comprises a (2:1) multiplexer 65 that multiplexes data signal regenerated by the regeneration circuits 64A and 64B and a receiving data processing part 66.
Specifically, the delay interferometers 62A and 62B receive the optical signals obtained by two-branching the (CS) RZ-DQPSK signal transmitted via the transmission line 50 and the optical repeater 51 by the branch part 61. The delay interferometer 62A performs delay interference between a delay component corresponding to one-bit time (46.5 ps in the structure example in FIG. 29) and a component subjected to the phase control with π/4 rad, and outputs the interference results as two signals.
Further, the delay interferometer 62B performs the delay interference between the delay component corresponding to one-bit time and a component (with the phase deviated from the delay component of the delay interferometer 62A with π/2 rad) subjected to the phase control with −π/4 rad, and outputs the interference results as two signals. Herein, the delay interferometers 62A and 62B individually comprise Mach-Zehnder interferometers, and dual-pin photodiodes that perform balanced detection by receiving the outputs, respectively. Incidentally, the receiving signals detected by the photo-electronic converting parts 63A and 63B are properly amplified by an amplifier.
The regeneration circuit 64A regenerates In-phases I of the clock signal and data signal from the receiving signal subjected to the balanced detection by the photo-electronic converting part 63A. Further, the regeneration circuit 64B regenerates Quadrature-phases Q of the clock signal and data signal from the receiving signal subjected to the balanced detection by the photo-electronic converting part 63B.
The (2:1) multiplexer 65 receives the In-phases I and the Quandature-phases Q from the regeneration circuits 64A, and 64B, and converts the received phases into data signals with 43 Gbit/s before the DQPSK modulation. The receiving data processing part 66 performs signal processing such as error correction on the basis of the data signal from the (2:1) multiplexer 65.
As mentioned above, from the market, a wavelength multiplexing transmission system is demanded, in which a phase modulation signal (signal modulated by the (CS) RZ-DQPSK modulation or (CS) RZ-DPSK modulation) with 40 Gbit/s and a conventional intensity modulation signal (signal modulated by NRZ modulation) with 10 (2.5) Gbit/s are mixed.
In this case, there is a problem that the phase modulation signal is subjected to optical phase shift with XPM (Cross Phase Modulation) from the intensity modulation signal, a waveform of the phase modulation signal obviously deteriorates, and the transmission at a long distance is not possible.
That is, even if mixing the phase modulation signal and the intensity modulation signal on the same network, it is a serious problem to prevent the waveform degradation (XPM degradation) of the phase modulation signal.